1. Field of the Invention
This invention concerns a DC bias device for radio frequency transmission systems of high power and low intermodulation, comprising resonator circuits made up of stubs. More specifically, the invention relates to a BIAS-T system comprising a radio frequency (RF) transmitting station or source and a receiving station for the aforementioned RF which must be activated by DC power coming from an outside source and injected on the line between the two stations.
2. Description of the Related Art
A DC bias device is a circuit for introducing DC power from an outside source to a part of an RF circuit without altering its function. The working principle of a conventional DC bias device is illustrated schematically in FIG. 1. A radio frequency circuit is shown, consisting, for example, of an RF-signal transmitting station A, and an RF-signal receiving station B. A DC bias comes from a power source D. A block S ideally must be a short circuit in series for the RF signal, and block P an open circuit in series for the DC signal.
FIG. 2 shows emblematically the simplest embodiment of the scheme of FIG. 1: a capacitor C works as a high-pass filter, and an inductance L (in series with D) works as a low-pass filter, whereby a DC signal introduced in D can reach B through the co-axial line without involving A. The circuit of FIG. 2 can be used at relatively low frequencies, on the order of hundreds of MHz, and for power below tens of watts.
When, however, the frequency exceeds, for example, one GHz, and the power runs to hundreds of watts, the devices of the type shown in FIG. 2 are unsuitable and do not satisfy practical requirements, giving rise to various inconveniences, due to which more complex circuits are needed comprising specific elements such as resonators. Such resonators can be made, for instance, with stubs on the two branches of FIG. 2.
A stub generally is a co-axial short- or open-circuit. The device shown in FIG. 2, which is a co-axial version, can incorporate two stubs, and have a structure as shown in the equivalent schemes of FIGS. 3 and 4.
In these figures, an open-circuit resonator RS1 in series with A is made of an inductor L1 and a capacitor C1 in series with L1. The short-circuit resonator RS2 is made of L2 and C2 connected in parallel. In FIG. 4, the RS1 and RS2 resonators reach the junction N positioned on the co-axial line 1 between A and B. The RS1 and RS2 resonators must keep the impedance matched on the AB line.
Being a co-axial line, the resonators can take on the form of stubs with cavities, for example, in the inner co-axial conductor, formed respectively of an external co-axial conductor of a branch (for instance A) in which the cavity is formed, into which cavity is inserted a protuberance of the inside co-axial conductor of the branch B.
FIG. 5 shows a classic, schematic, partial cross-section of a resonator RS1 with a stub STU1. CE1 is the outer conductor of the co-axial line 1 between A and B. The inner co-axial conductor CI(A) has a cylindrical cavity CC, and an opening AP through which is inserted the protuberance PR1 of the inner conductor CI(B) of the branch B of the co-axial line 1 between A and B. The cavity CC is closed at its other extremity by a bottom 61.
The cavity CC in the internal conductor CI(A) and the protuberance PR1 of the inside conductor CI(B) together form a stub STU1 terminated in an open circuit (W-Z). In a narrow band, stub STU1 (forming the RS1 resonator) can be equivalently represented by the circuit L1, C1 of FIG. 3.
FIG. 6 shows another possible stub STU2 terminated with a short circuit for the RF and also formed by a cavity and a protuberance. The inner conductor CI2 (B) shows a protuberance PR2 which is closed in a short circuit on the bottom X-X' of the cylindrical cavity PCC connected to the outer conductor CE2 of the branch AB.
For the best functioning of the two stubs STU1 and STU2, taken singularly, it is recommended that the STU1 open-circuit stub in series on A-B (FIG. 5) have a characteristic impedance which is the lowest possible, in order to have, among other things, the least reflection.
If, however, the two stubs are used together, that is, combining the aforementioned stubs STU1 (series) and STU2 (parallel), to combine the features specified above, among other drawbacks, the following disadvantages which are extremely problematic to the combination occur:
the two reflections not only are not eliminated but are even free to add to each other; and PA1 even if there was a matching, the compensation which might result would always be only partial. PA1 1. Giving to the parallel, short-circuit stub STU2 a characteristic normalized impedance Z' op equal to the characteristic normalized admittance of the series, open-circuit stub STU1, the admittance being the highest possible, compatible with the realization constraints; and PA1 2. Putting the near ends of the two stubs at the lowest possible distance, in particular at a distance below .lambda./100 (where .lambda. corresponds to the central frequency F0 of the RF signal), the maximum of such a distance being below .lambda./20.
Accordingly, the need exists for a DC bias device which does not have the aforementioned impediments, has simple constructional design, and is very reliable.